So what really happened?

My efforts to vindicate the crew of TWA 841 through a thorough examining of the evidence would not be complete if I didn’t also put forth evidence explaining the most likely cause of the upset. The book and possible documentary will lay out the facts of the case in, I hope, an impartial manner so that the reader can draw their own conclusions.

But it is also important to provide evidence that supports the alternative theory that the cause of the upset was not an extended slat but instead a discrepant rudder. This post will highlight a number of cases involving uncommanded roll caused by rudder/yaw damper system malfunction.

Interestingly enough, this first case below occurred a little over two months prior to TWA 841. The incident involved a 727 on a flight from Laguardia airport in New York to Atlanta, Georgia on January 21, 1979. The pilot reported rudder control problems. Here the pilot describes what happened shortly after taking off from Laguardia with thunderstorms in and around the area. (Morrill, C. William. Flight 201, January 21, 1979 flight summary. January 22, 1979.)

“…At 400’ I felt a skid to the left which I attributed to expected wind shear. We turned to 170 degrees. Around 800’another skid was felt. We were in heavy rain and considerable turbulence at the time. This time the skid seemed to increase and hold. I thought we had lost an engine and made the comment to my crew. The reply was everything was normal. My thoughts were this was the most unusual wind shear I have ever experienced as it was apparently overcoming the 727 yaw system.

“We were instructed to turn right to 240 degrees. It was then apparent that there was a problem with the aircraft, as I had full aileron input in to the right and the aircraft was in a left skid and not turning. ATC made the request again to turn. I told my crew that I would fly the airplane and for them to check the systems. Someone announced to look at the rudder indices as they were split in opposite directions between the yaw damper and full travel pointers. The aircraft then went into a 35 degree right bank as apparently the rudders went to the right. I was controlling the aircraft with aileron, as I did not want to aggravate the rudder problem.

“At this time the aircraft was in a right bank with considerable left aileron input. It stopped and I had positive control of the aircraft again. I then made a turn to the west experiencing several rudder inputs in the turn.”

The pilot goes on to explain how they eventually determined that the problem was related to the upper rudder system. Once they turned off the upper rudder yaw damper the problem went away. The pilot was describing a rudder hardover of the upper rudder. Had the hardover occurred on the lower rudder, which has more surface area, they might not have been able to recover. Another factor in their favor was their low altitude.

This was an important clue that somehow was overlooked by the ALPA investigators looking into TWA 841, most likely because all the focus was on the #7 slat. The report was filed away without any further investigating.

Since TWA 841 there have been a number of similar cases of uncommanded roll due to yaw damper system malfunction. Below is an aircraft service report involving Eastern Airlines Flight 56 on January 3, 1982. (Aircraft Incident Report. Eastern Airlines Flight 56. January 3, 1982.)

TWA Flight 70, N5430, a Boeing 727-231, departed St. Louis (STL) on 12/16/89 2326 CST en route to Newark (EWR). At approximately 40 nautical miles NW of Dayton (DAY) at FL 370, mach .825, the crew suddenly experienced uncommanded aileron and rudder input with the autopilot engaged. The crew reported they needed full right input of the autopilot to maintain level flight. They then disconnected the autopilot. The crew then reported that the aircraft required major forceful control inputs to maintain control of the aircraft. Captain Martinex declared an emergency to (IND) center and then diverted to Dayton Int’l (DAY). During approach into Dayton, the crew observed a split flap indication. The aircraft then made an uncontrolled turn to the left. The crew then immediately retracted the flaps. The crew made a no flap landing. The crew also reported elevator inputs were severely restricted in a no flap landing condition and that it was difficult to flare the aircraft. The aircraft was ferried to Kansas City for maintenance. TWA maintenance accomplished the following: (1) Replaced both upper & lower rudder power control units. (2) Re-rigged rudder control system. (3) Replaced both autopilot yaw damper couplers. (4) Checked flap position indicators. (5) Re-rigged flap control system. (6) Test flew aircraft. The aircraft was released to resume service on December 29, 1989. There have been no further maintenance discrepancies reported.

Here is a report of a rudder induced uncommanded roll that occurred on March 12, 1991. (Marthinsen, Harold. Letter to Leroy A. Keith FAA. June 5, 1991.)

At cruise FL 280 aircraft yawed to the right and the autopilot corrected with left aileron. At that time we noticed the upper rudder indices showed approximately 25 per cent deflection to the left and the lower rudder indices showed approximately 50 per cent deflection to the right with the autopilot still engaged the upper and lower rudder returned to center and everything appeared normal with [the] autopilot disconnected. The aircraft felt normal also.

90 DME east [of] ZUN, 28,000, 265 kts, autopilot coupled, VMC conditions (visual meteorological conditions), aircraft yawed to right (indices approximately 20 per cent right). I disconnected the autopilot and maintained wings level with 30 to 40 degrees left aileron. After 6 seconds rudder returned to normal condition. Fifteen minutes later both rudder indices went to left (lower rudder leading upper). Autopilot again disconnected.

None of the incidents above resulted in loss of control. But none of them occurred under the same flight conditions as TWA 841, which was at cruise at 39,000 feet, mach .80, with an outboard right aileron suspected of upfloating, restricting the amount of lateral control available for recovery.

A look through the accident record of the Boeing 727 reveals that there have not been any unexplained accidents that might have been attributable to a malfunctioning rudder/yaw damper system. But the evidence clearly shows that the 727 was and is susceptible to discrepant rudder input.

Trackbacks

[…] There is plenty of evidence to suggest that the slat had nothing to do with the upset but was instead lost during the recovery. Additionally, there are a number of incidents on record involving yaw damper system malfunction. See the post So what really happened. […]